This invention relates to an optical system which changes the amplitude distribution profile of a laser light beam from a Gaussian distribution profile to a (sin x)/x distribution profile and changes the intensity distribution profile of the same laser light beam from a Gaussian distribution profile to a [(sin x)/x].sup.2 distribution profile. In referring to (sin x)/x distribution and [(sin x)/x].sup.2 distribution, x is the distance measurement from the beam center in the beam cross section. Hereinafter (sin x)/x is referred to as "sinc function" and [(sin x)/x].sup.2 is referred to as "sinc squared function". More specifically, this invention relates to a raster output scanner in which the amplitude distribution profile and the intensity distribution profile of the light beam are reprofiled by a pair of binary diffraction optic lenses to provide a square pixel profile in both amplitude and intensity at the photoreceptor plane. In the raster output scanner of this invention, the laser light beam is reprofiled prior to impacting a rotating polygon mirror.
Referring to FIG. 1, a conventional raster scanner system 10 utilizes a light source 12, a collimator 14. Pre-polygon optics 16, a multi-faceted rotating polygon mirror 18 as the scanning element, post polygon optics 20 and a photosensitive medium 22. The light source 12, which can be a laser source, produces a light beam 24 and sends it to the rotating polygon mirror 18 through the collimator 14 and the pre-polygon optics 16. The collimator 14 collimates the light beam 24 and the pre-polygon optics 16 focuses the light beam in the sagittal or cross-scan plane onto the rotating polygon mirror 18. The rotating polygon 18 has a plurality of facets 26, each of which is a plane mirror. However, in the tangential or scan plane, the collimated light beam passes through the pre-polygon optics without being altered and it strikes the polygon as a collimated light beam.
The facets 26 of the rotating polygon mirror 18 reflect the light beam 24 and also cause the reflected light 24 to revolve about an axis near the reflection point of the facet 26 of the rotating polygon mirror 18. This reflected light beam can be utilized through the post polygon optics 20 to scan a document at the input end of an imaging system as a raster input scanner (RIS) or can be used in a raster output scanner (ROS) to impinge upon a photographic film or a photosensitive medium 22, such as a xerographic drum (photoreceptor), at the output of the imaging system.
Referring to FIG. 2, typically, a laser light beam has two Gaussian distribution profiles: one for intensity 30 and one for amplitude 32. In FIG. 2, the horizontal axis represents the distance measurement x from the beam center C in the beam cross section and the vertical axis represents intensity and amplitude. Intensity is defined as the number of photons/cm.sup.2 /sec and the amplitude is defined as the voltage field in the light wave.
Furthermore, intensity 30 is equal to squared amplitude 32 [Intensity=(Amplitude).sup.2 ]. It should be noted that the bell shape profile of both amplitude and the intensity Gaussian distributions are continued throughout the optical elements in a raster scanning system.
Typically, based on the intensity distribution and the threshold level at the photoreceptor, the spot size at the photoreceptor can be defined. However, the bell shape profile of a Gaussian intensity distribution causes a problem in multilevel xerographic systems. In multilevel xerographic systems, for each color, there is a separate threshold level. Depending on the threshold level, the maximum intensity of the light beam has to be changed. This causes the spot size for different colors to be different.
For example, referring to FIG. 3, different threshold levels 33, 34 and 35 are assigned to three levels of a tri-level printing system. Also, in FIG. 3, there are shown three Gaussian distributions 36, 37 and 38 each having a different maximum intensity. In FIG. 3, the horizontal axis represents the distance measurement x from the beam center C in the beam cross section and the vertical axis represents the intensity. In this system the maximum threshold 33 is assigned to black, a mid-range threshold 34 is assigned to white and a low threshold 35 is assigned to color.
For each threshold level 33, 34 and 35, the maximum intensity of the light beam is changed. For threshold level 35, the maximum intensity is at I.sub.MAX1. If the threshold level 34 is needed, the the intensity of the light beam will be changed to I.sub.MAX2. Eventually, for the threshold level 33, the maximum intensity of the light beam will be changed to I.sub.MAX3. It should be noted that while the intensity of the light beam is changed, the base B of the Gaussian distribution is kept the same, as defined by a fixed beam radius to the 1/e.sup.2 intensity value.
Due to the bell shape profile of the light beam, the beam width at different threshold levels will be different. Thus for the Gaussian distribution 36, at threshold level 33, the beam width is a, at threshold level 34, the beam width is b and finally at threshold level 35 the beam width is e. The variation of the beam, width at different threshold levels causes a spot size variation for different colors. This is a particular problem in multilevel xerographic systems where pixel color depends upon the beam intensity.
Therefore for the Gaussian distribution 36 of FIG. 3, referring to FIG. 4, the black level creates a spot with a diameter equal to a, the white level creates a spot with a diameter equal to b and the color level creates a spot with a diameter equal to e. This causes a multicolored halos around each spot or pixel.
In addition, if the output power of the laser diode changes, the Gaussian intensity distribution changes and therefore the spot sizes of the different colors also change.
It should be noted that even if one color is used meaning that one threshold level is selected, by variation of the output power of the laser diode, the maximum intensity changes and as a result, the spot size changes.
Referring to FIG. 5, It is an object of this invention to provide a square or approximately square intensity profile at the photoreceptor plane in which the width of the light beam d at different threshold levels 33, 34 and 35 stays the same or substantially the same. With a square intensity profile, even if the power output of the laser diode fluctuates, the beam width for different colors stays the same. It should be noted that a light beam with square intensity profile will inherently have a square amplitude profile. Hereinafter, when the terms "square intensity profile" and "square amplitude profile" are used, such terms shall imply that the beam width at different threshold levels will be substantially the same.